Assuming that disk cost is the amount of drive space allocated for overhead and not the number of disks lost to that overhead (I'm still not too clear on that from your explanations), you can calculate the disk cost with the simple equation C = D / n, where C is the disk cost in drive space, D is the total drive space available, and n is the number of disks.

Thus for an array of 3 500 GB disks, the cost would be calculated by

C = 1500GB / 3 = 500GB